The present invention relates to a sheet convey apparatus for a sheet-fed offset rotary printing press with a convertible press mechanism which prints on the obverse and reverse surfaces of a sheet by reversing the sheet by a convertible cylinder.
Generally, in a sheet convey apparatus of this type, the sheets chucked from a feed device to the suction port of a chucking device one by one are fed onto a feeder board as a feed roller rotates. The paper sheets on the feeder board are conveyed to a feedboard by a feed tape or the like, and the leading edges of the paper sheets fed onto the feedboard are jogged by a register. The paper sheets with the jogged leading edges are each gripped by the grippers of a swing arm shaft pregripper. When the swing arm shaft pregripper rotates, each paper sheet is conveyed to the cylinder of a printing press and transferred to the grippers of the cylinder. The paper sheet gripped by the grippers of the cylinder is printed while it passes through the contact point between an impression cylinder and blanket cylinder.
Moisture, high pressure, and the like which are supplied when the paper sheet is to be printed cause a phenomenon that the trailing edge of the paper sheet is stretched in the left and right. When this stretch occurs, the printing register, which is called fan-out register, of the non-gripped edge of the paper sheet is undesirably shifted during multicolor printing to cause a printing error.
In order to solve this problem, in the conventional sheet convey apparatus, when a paper sheet is to be transferred from the upstream side in the sheet convey direction, a gripper pad and gripper shaft are flexed in the sheet convey direction, as shown in U.S. Pat. Nos. 5,505,441 and 6,283,467. When the paper sheet is to be transferred to the downstream side in the sheet convey direction, the flex of the gripper pad and gripper shaft in the sheet convey direction is removed. Then, when the two ends of the paper sheet are pulled by grippers corresponding to the two ends of the paper sheet, the paper sheet can be transferred with its trailing edge being kept taut.
In the conventional sheet convey apparatuses described above, the paper sheet transferred from a sheet holding means on the upstream side in the sheet convey direction is transferred to a downstream sheet holding means without being reversed. Thus, this sheet convey apparatus cannot be applied to a convertible cylinder that reverses the obverse and reverse sides of the paper sheet by a pair of converting grippers provided to one cylinder. If the conventional sheet convey apparatus is to be applied to a sheet-fed offset rotary printing press with a convertible cylinder having a structure of this type, a fan-out register adjusting cylinder and convey cylinder must be arranged between the convertible cylinder and a cylinder located downstream of the convertible cylinder, and the entire length of the printing press increases accordingly. If a register adjusting mechanism is to be provided to the convertible cylinder, the structure of the convertible cylinder itself becomes complicated.